The present invention relates to a method for stabilizing nitrile resins against discoloration resulting from thermal degradation during processing. More particularly this invention relates to the stabilization of polymers and graft polymers of acrylonitrile to prevent thermal discoloration by the inclusion of a minor amount of an ethylenically-unsaturated amide or ester. Still more particularly, this invention relates to the stabilization of emulsion-polymerized polymers and graft polymers of acrylonitrile by the inclusion of a minor amount of an ethylenically-unsaturated amide or ester together with minor amounts of a Group IIA metal salt.
Acrylonitrile polymer and copolymer resins are generally regarded as being materials of good thermal stability. They are, however, subject to thermal discoloration, and thermal processing of the unstabilized resins frequently results in development of yellow, amber and even reddish casts during the extrusion of films and fibers. Consequently a substantial effort has been made to develop methods of reducing or eliminating thermal discoloration to produce clear, color-stable films and molded articles. Prior art methods for stabilizing acrylonitrile resins against thermal discoloration include such processes as adding small amounts of such compounds as maleic acid, dialkyltin maleates, maleimide and the like as color stabilizers. More recently, the addition of minor amounts of acrylamide to acrylonitrile resins has been shown to provide improved stability against thermal discoloration, such as is shown in U.S. Pat. No. 3,925,305. Related acrylamides, however, and in particular N-substituted acrylamides, have been reported as ineffective for the purpose of stabilizating nitrile resins. Certain acrylates have also been employed to stabilize polyacrylonitrile against gel formation. The use of polyfunctional acrylates to stabilize nitrile resins has not been reported.